Protection of a non-volatile memory by change of instructions

ABSTRACT

A method for protecting a volatile memory against a virus, wherein: rights of writing, reading, or execution are assigned to certain areas of the memory; and a first list of opcodes authorized or forbidden as a content of the areas is associated with each of these areas.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of French patent application number 11/53804, filed on May 4, 2011, entitled PROTECTION OF A NON-VOLATILE MEMORY BY CHANGE OF INSTRUCTIONS, which is hereby incorporated by reference to the maximum extent allowable by law.

BACKGROUND

1. Technical Field

Embodiments generally relate to electronic circuits and, more specifically, to circuits equipped with volatile memories, be they temporary storage RAMs or processing unit execution stacks.

Embodiments more specifically relate to the protection of such volatile storage elements against virus attacks.

2. Discussion of the Related Art

Whether in a RAM where a program is stored for execution or in a dynamic processing unit stack where the instructions are successively placed for the execution of this program, the corresponding memory areas are particularly sensitive to virus-type attacks.

Two main categories of viruses are known.

Viruses of a first category take the place of the program, that is, modify the opcodes (and sometimes also the arguments) of some instructions so that the program carries out another task than that for which it is initially provided.

Viruses of a second category can be considered as external to the executed program and have the function of modifying the data, that is, the values of the arguments manipulated by the legitimate program, especially when said arguments are stored in the program execution stack.

Countermeasures which affect rights of writing, reading, or execution on files or memory areas are known. Thus, if a virus attempts to carry out a forbidden action of the concerned memory area, it is not allowed to do it. However, such measures are often considered as insufficient. In particular, an area where it is usual to execute a program is accessible in write and in execution mode. A virus can thus replace, in this area, a program or a program portion and execute it. Similarly, a virus can modify the data contained in the execution stack.

SUMMARY

An embodiment aims at overcoming all or part of the disadvantages of protections of volatile memories against viruses.

According to a first aspect, an embodiment provides a solution more particularly intended for a virus tending to modify the instructions of a program.

According to a second aspect, an embodiment provides a solution more specifically intended for a virus, external to an executed program, aiming at replacing instruction operands.

Thus an embodiment provides a method for protecting a volatile memory against a virus, wherein:

rights of writing, reading, or execution are assigned to certain areas of the memory; and a first list of opcodes authorized or forbidden as a content of the areas is associated with each of these areas.

According to an embodiment, said list is stored at the same address as the concerned area.

According to an embodiment, said list is stored in a table separate from the concerned area.

According to an embodiment, the volatile memory is a RAM.

According to an embodiment, the volatile memory is a stack associated with a processing unit.

According to an embodiment, a second list of opcodes for which the access to the areas is authorized or forbidden is associated with each of these areas.

Another embodiment provides an electronic circuit capable of implementing the above method.

The foregoing and other objects, features, and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a very simplified block diagram of an example of an electronic system of the type to which the embodiments which will be described apply as an example;

FIG. 2 illustrates an example of a dynamic instruction storage stack associated with a processing unit;

FIG. 3 illustrates in simplified fashion an example of the content of a RAM-type volatile memory; and

FIG. 4 is a simplified representation of an embodiment of a method for protecting a volatile memory.

DETAILED DESCRIPTION

The same elements have been designated with the same reference numerals in the different drawings. For clarity, only those elements and steps which are useful to the understanding of the embodiments have been shown and will be described. In particular, the destination of the protected programs or data has not been detailed, the embodiment being compatible with usual destinations of such programs to be protected against viruses. Further, the content of the volatile memories targeted by the described embodiments has only been detailed where this clarifies the discussion.

FIG. 1 is a very simplified representation of an embodiment of an electronic circuit to which the embodiments apply as an example. Circuit 1 comprises a processing unit 11 (CPU) associated with one or several non-volatile memories 12 (NVM) and with one or several RAMs 13, as well as one or several instruction stacks generally contained in a RAM. The different elements of circuit 1 communicate together and with a wire or contactless input/output interface 14 (I/O), via a data, control, and address bus 15. Circuit 1 may comprise other elements symbolized by a block 16 (FCT) performing other functions (for example, a cryptoprocessor, sensors, other electronic elements, etc.).

Central processing unit 11 comprises registers (not shown in FIG. 1) into which the instructions being executed are temporarily transferred. These instructions generally originate from RAM 13 into which programs for example contained in non-volatile memory 12 or originating from the outside of circuit 1 via interface 15 are transferred to be executed.

FIG. 2 is a very simplified representation of an execution sequence 21 associated with processing unit 11. Each instruction (INST) is most often formed of an opcode (OPCODE) associated with arguments (ARG). The arguments are either addresses or data processed by the opcode. The number of arguments depends on the concerned instruction. The execution sequence is accessed by the processing unit.

Some instructions in the sequence aim at manipulating the program execution stack. One can in particular find the following instructions:

PUSH to stack a piece of data (value or address);

POP to pop a piece of data (value or address);

CALL to stack, in particular, the return address of the calling function, that is, that which performs the CALL, and jump to the address of the called function; and

RET to pop a return address and jump to it.

FIG. 3 is a very simplified representation of a RAM memory plane 23. This memory plane comprises one or several programs (P1 and P2 in the example of FIG. 3) which are transferred, for their execution, either from a non-volatile memory, or from the outside of the circuit. This program is then accessed by the central processing unit. The execution stack is generally stored in this memory.

The elements of FIG. 2 and of FIG. 3 will be referred to as volatile memory elements. Such volatile memory elements are likely to be attacked by computer viruses.

In the embodiments which will be described, it is provided to assign authorized opcodes (OPCODE) to volatile memory areas (execution stack or RAM). Such a solution adds a granularity to usual protections, which only assign rights in reading, writing, or execution to these memory areas.

FIG. 4 illustrates an embodiment according to which, to each address 31 (ADDR) of the volatile memory pointing to a memory content area 32 (AREA), are assigned not only one or several rights 33 (RIGHT) among rights of execution, reading, or writing, but also one or several lists 34, 35 (LIST I, LIST II) of authorized opcodes.

Each memory area is thus “tattooed” with the list of authorized instructions for this area, relatively to the program destination, that is, relatively to the semantics of the executed function. For example, if it is an area of arithmetic calculation to be executed linearly, all arithmetical opcodes will be authorized, but jumping operations will be forbidden.

The nature of the opcodes depends on the electronic circuit and on the language used by the processing unit. Further, although these codes have been expressed more or less intelligibly, they of course in practice are digital codes.

According to the first aspect, the authorized or forbidden opcodes concern the content of area 32.

The fact of adding, to memory areas identified by their addresses, opcodes authorized or forbidden as a content of the addressed areas, prevents a virus external to the program which would be downloaded into a memory area or into the processing unit stack, to efficiently replace program instructions. Indeed, although a replacement of the instructions by this virus remains possible, if these instructions contain a non-authorized opcode on the corresponding memory area, the access is denied and the virus becomes inefficient. To counteract this protection, the virus would have to be able to identify the authorized opcodes in the area where it desires to write. This becomes particularly complicated and non-profitable for an attacker.

In a simplified embodiment, it may be provided for a RAM to have, according to areas, set authorized opcodes in the different addresses. This, however, limits the possibilities of using a same memory area for different programs.

Thus, in one embodiment, the opcodes assigned to the different memory areas are capable of being modified for each loading of a program into the memory area.

The list of authorized or forbidden codes is stored, for example, in additional memory words or registers at the addresses of the concerned areas. As a variation, a separate table storing, for each address or address area, the list of authorized and forbidden opcodes is provided.

A coding of the groups of opcodes may also be provided to assign a same value to a set of licit or illicit operations for a same memory area.

According this aspect, a countermeasure intended for a virus which will modify the instruction operands of a program to use this instruction for other purposes is provided.

The countermeasure discussed in relation with the first aspect is not sufficiently efficient since the opcode associated with the address at which the instruction is located is, in this case, actually authorized.

According to the second aspect, the memory areas are assigned a list of opcodes capable of accessing this area. As compared with the first mode, the case in point is not to authorize or to forbid for the area to contain such and such an opcode in an instruction, but to authorize or forbid for the information contained in the area to be accessed by such and such an opcode.

As for the first aspect, this countermeasure comes as a complement to the rights of access in reading, writing, or execution, to the concerned area.

Without the first countermeasure, a virus may modify instructions in memory areas to replace such and such a program. Further, an external virus modifying the argument of an instruction may start being executed. However, if the address introduced by the virus as an argument of the instruction points to a memory area where the instruction (its opcode) is not authorized by the countermeasure of the second aspect, the value of the area is not affected. Accordingly, the virus has no effect.

The memory portions likely to being vulnerable to an external virus are thus considerably limited.

As in the first aspect, implementing the countermeasure of the second aspect requires associating, with each address or group of addresses, a list of authorized or forbidden opcodes (block 35, LIST II, FIG. 4). The difference is that it lists the codes having or not the right to access area 32 while LIST I lists the codes authorized or forbidden in area 32.

Still as in the first aspect, the definition of the authorized or forbidden opcodes may, in a simplified embodiment, be set, or in a more advanced implementation, be updated on each loading of the program into the memory.

An advantage of the described embodiments is that they add a protection to existing solutions. The two above-described aspects may of course be combined.

Various embodiments have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. Further, the practical implementation of the described embodiments is within the abilities of those skilled in the art based on the functional indications given hereabove. The price to pay to implement these embodiments is a table or an additional memory area containing the authorized or forbidden opcodes for each of the areas.

Further, the action to be taken after a rule violation (attempt of access to a non-authorized area or to write a non-authorized code into an area) may vary according to applications. It may be provided to block the circuit, to warn the user, or to simply reject the instruction, etc.

The granularity desired for the memory areas to be protected depends on the application. This granularity may be word per word, word group per word group, etc. The finer the granularity, the better the protection, but the more additional memory is consumed.

Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto. 

1. A method for protecting a volatile memory against a virus, wherein: rights of writing, reading, or execution are assigned to certain areas of the memory; and a first list of opcodes authorized or forbidden as a content of the areas is associated with each of these areas.
 2. The method of claim 1, wherein said list is stored at the same address as the concerned area.
 3. The method of claim 1, wherein said list is stored in a table separate from the concerned area.
 4. The method of claim 1, wherein the volatile memory is a RAM.
 5. The method of claim 1, wherein the volatile memory is a stack associated with a processing unit.
 6. The method of claim 1, wherein a second list of opcodes for which the access to the areas is authorized or forbidden is associated with each of these areas.
 7. An electronic circuit capable of implementing the method of claim
 1. 